1. Field of the Invention
The present invention relates to a multi-input multi-output (multiple antennas) mobile communication system. More particularly, the present invention relates to an apparatus and a method for improving the performance of an error correction code in response to the influence of error propagation.
2. Description of the Related Art
A conventional mobile communication system provides a voice-service and mainly uses channel coding to overcome unfavorable channel conditions. However, a multimedia service of high quality increases the necessity for a next generation wireless transmission technology that can transmit a large quantity of data with few errors. In particular, high speed transmission is more important in a forward link having a high transmission quantity of data. However, in a mobile communication system, the reliability of a signal is largely reduced due to fading, shadow, wave attenuation, noise, interference, etc. In particular, fading due to a multi-path causes severe signal distortion due to the sum of signals which are received through different paths and have different phases and sizes. Since the fading must be overcome to support high speed data communication, research into the fading has been actively pursued. Accordingly, a multi-input multi-output (‘MIMO’) technology using a plurality of transmission/reception antennas has been proposed. The MIMO simultaneously transmits data to a transmitter and a receiver by means of multiple antennas, thereby transmitting a large quantity of data without increasing transmission bandwidth.
FIG. 1 is a block diagram illustrating a conventional MIMO system. As shown in FIG. 1, a transmitter includes a demultiplexer 100, a signal processor 102 and transmission antennas 104, 106 and 108 and a receiver includes reception antennas 110, 112 and 114 and a signal processor 116. FIG. 1 shows only elements necessary for describing the principle of the MIMO system. Further, a plurality of inter-antenna channels are formed between the transmission antennas 104, 106 and 108 and the reception antennas 110, 112 and 114.
Referring to FIG. 1, the demultiplexer 100 demultiplexes a transmitted data stream into the same number of data streams as the number of the transmission antennas 104, 106 and 108, and outputs the multiplexed data streams. That is, the demultiplexer 100 duplicates each of the transmitted user data streams into the same number of data streams as the number of transmission antennas. Each of the user data streams is overlappingly transmitted through a multiple antenna in this manner, so that the error probability for the user data stream is reduced. Therefore, the reliability of the received user data stream can be improved. In other cases, the demultiplexer 100 receives the same number of data as the number of antennas and can output the received user data streams to transmission antennas.
The user data streams sent from the demultiplexer 100 experience a predetermined processing by the signal processor 102 and are then output to the transmission antennas 104, 106 and 108. The transmission antennas 104, 106 and 108 transmit the received user data streams to the reception antennas 110, 112 and 114. The reception antennas 110, 112 and 114 receive the user data streams transmitted from the transmission antennas 104, 106 and 108. That is, the reception antenna 110 receives the user data stream transmitted from the transmission antennas 104, 106 and 108, and the reception antenna 112 receives the user data stream transmitted from the transmission antennas 104, 106 and 108. Similarly, the reception antenna 114 receives the user data stream transmitted from the transmission antennas 104, 106 and 108. The reception antennas 110, 112 and 114 sends the received user data streams to the signal processor 116. The signal processor 116 performs a predetermined processing such as coding and modulation for the received user data streams.
The MIMO system may employ a bell labs layered space-time (‘BLAST’) scheme and a per-antenna rate control (PARC) scheme. Hereinafter, the BLAST scheme will be first described.
A transmitter of the BLAST scheme demultiplexes a user data stream into the same number of data streams as the number of transmission antennas and the transmission antennas use the same data rate. The BLAST scheme may be classified into a diagonal BLAST (‘DBLAST’) scheme, a vertical BLAST (‘VBLAST’) scheme and a horizontal BLAST (HBLAST) scheme. The DBLAST scheme performs a specific block coding for a user data stream transmitted from each transmission antenna, thereby improving efficiency. However, it is difficult to realize the DBLAST scheme. The VBLAST scheme performs an independent coding for a user data stream transmitted from each transmission antenna. In such a VBLAST scheme, the number of reception antennas is equal to or larger than that of transmission antennas and a receiver uses a maximum likelihood detection (‘ML’) scheme. In the ML scheme, symbols having a minimum error are selected through substitution of all symbols transmittable in all transmission antennas, so as to greatly improve performance of the antennas.
However, since the calculation amount increases due to the increase of the number of the transmission antennas, it is difficult to realize the ML scheme.
Meanwhile, the PARC scheme assigns data rates differently according to channel states experienced by each transmission antenna. The channel state may be expressed by a signal-to-interference and noise ratio (SINR).
FIG. 2 is a block diagram showing a structure of a transmitter of a MIMO system using a PARC scheme. FIG. 2 shows a system capable of simultaneously transmitting J×M user data streams by means of J spreading codes and M transmission antennas.
The user data stream is transmitted to a demultiplexer 200. The demultiplexer 200 divides the user data stream by the J number of data according to the number of the transmission antennas and sends the divided user data streams to signal processors 210, 212 and 214. The signal processors 210, 212 and 214 perform a predetermined signal processing for the received user data streams.
Further, the signal processors 210, 212 and 214 perform a coding, an interleaving, a modulation, etc., for the received user data streams by means of preset data rates, respectively. The signal processors 210, 212 and 214 send the processed user data streams to spreaders 220, 222 and 224. Herein, the J output data streams processed by the signal processor 210 are respectively output to the spreaders 220, 222 and 224. Similarly, the signal processor 212 outputs the J output data streams to the spreaders 220, 222 and 224 and the signal processor 214 outputs the J output data streams to the spreaders 220, 222 and 224.
The spreaders 220, 222 and 224 use different spreading codes. The spreader 220 performs spreading for the user data streams sent from the signal processors 210, 212 and 214 by means of the same spreading code 1, the spreader 222 performs spreading for the user data streams sent from the signal processors 210, 212 and 214 by means of the same spreading code 2, and the spreader 224 performs spreading for the user data streams sent from the signal processors 210, 212 and 214 by means of the same spreading code J.
The user data streams that experienced the spreading by the spreaders 220, 222 and 224 are output to adders 230, 232 and 234. Herein, the user data streams (having experienced the coding/interleaving/modulation) processed by the same signal processor are output to the same adder. Specifically, the user data stream processed by the same signal processor 210 is output to the adder 230, the user data stream processed by the same signal processor 212 is output to the adder 232, and the user data stream processed by the same signal processor 214 is output to the adder 234.
The data stream added by the adder 230 according to each antenna is subjected to an additional signal processing (i.e., frequency up-conversion) of the transmitter and is then transmitted through a radio channel by a first transmission antenna 240 as a signal S1(t). Herein, since the additional signal processing is not directly associated with the main scope of the present invention, a detailed description will be omitted. Next, the data stream added by the adder 232 according to each antenna is subjected to the additional signal processing of the transmitter and is then transmitted through a radio channel by a second transmission antenna 242 as a signal S2(t). Last, the data stream added by the adder 234 according to each antenna is subjected to the additional signal processing of the transmitter and is then transmitted through a radio channel by an Mth transmission antenna 244 as a signal SM(t).
FIG. 3 is a block diagram showing a structure of a receiver of an MIMO system using a PARC scheme. The structure of the receiver shown in FIG. 3 corresponds to the structure of the transmitter shown in FIG. 2.
Referring to FIG. 3, a reception antenna 300 receives the user data streams sent from the transmission antennas 240, 242 and 244. Referring to FIG. 2, the reception antenna 300 receives the signal sent from the transmission antennas 240, 242 and 244. Further, the reception antenna 302 receives the signal sent from the transmission antennas 240, 242 and 244 and the reception antenna 304 receives the signal sent from the transmission antennas 240, 242 and 244.
The reception antenna 300 sends the received signal to despreader 320 to 322, the reception antenna 302 sends the received signal to despreaders 323 to 325, and the reception antenna 304 sends the received signal to despreaders 326 to 328. Spreading codes used in the despreaders 320 to 328 are the same as those used in the spreaders 220, 222 and 224 of the transmitter. That is, the despreaders 320, the despreaders 323, the despreaders 326 and the spreader 220 of the transmitter use the same spreading codes. Further, the despreaders 321, the despreaders 324, the despreaders 327 and the spreader 222 of the transmitter use the same spreading codes. Similarly, the despreaders 322, the despreaders 325, the despreaders 328 and the spreader 224 of the transmitter use the same spreading codes.
The signal despreaded by the despreader 320 is output to a mean minimum square error (‘MMSE’) receiver 330, the signal despreaded by the despreader 321 is output to an MMSE receiver 332, the signal despreaded by the despreader 322 is output to an MMSE receiver 334, the signal despreaded by the despreader 323 is output to an MMSE receiver 330, the signal despreaded by the despreader 324 is output to an MMSE receiver 332, the signal despreaded by the despreader 325 is output to an MMSE receiver 334, the signal despreaded by the despreader 326 is output to an MMSE receiver 330, the signal despreaded by the despreader 327 is output to an MMSE receiver 332, and the signal despreaded by the despreader 328 is output to an MMSE receiver 334.
The MMSE receivers 330, 332 and 334 detect user data streams by a preset rule according to a spreading code of a specific transmission antenna. The detected user data streams of the specific transmission antenna are output to a multiplexer 340. The multiplexer 340 multiplexes the received user data streams of the specific transmission antenna and outputs the multiplexed data streams to a signal reverse-processor 350. The signal reverse-processor 350 detects the received data streams according to a preset antenna index sequence and performs a predetermined signal reverse-processing such as a demodulation, a deinterleaving, a decoding, etc. Herein, it is assumed that data streams are detected in a sequence of the first transmission antenna 240, the second transmission antenna 242 and the Jth transmission antenna 244. Accordingly, in the first step, the transmission signal of the first transmission antenna 240 is detected.
The data stream of the first transmission antenna 240 reverse-processed by the signal reverse-processor 350 is output to the next terminal 370. In addition, the reverse-processed data stream of the first transmission antenna 240 is output to a signal processor 360. The signal processor 360 performs the signal processing equal to that of the transmitter for the data stream of the first transmission antenna 240 sent from the signal reverse-processor 350. The signal processing includes a coding, an interleaving and a modulation. In this manner, the signal processing is performed, so that the transmission signal estimated as a signal transmitted from the first transmission antenna 240 is reconstructed.
The reconstructed transmission signal of the first transmission antenna 240 is output to subtracters 310, 312 and 314. The subtracters 310, 312 and 314 subtract the reconstructed transmission signal of the first transmission antenna 240 from the signal received in the reception antennas 300, 302 and 304, and provides the subtraction result to the despreader 320 to 328. The aforementioned process is repeatedly performed up to the transmission signal of the Jth transmission antenna. Therefore, the receiver can exactly receive the transmission signals sent from the transmitter while sequentially reducing the influence by the multiple transmission antenna.
In the conventional MIMO communication system as described above, a transmission signal of an Mth transmission antenna is estimated by means of an estimated transmission signal of an (M-1)th transmission antenna. A scheme of estimating a transmission signal of a transmission antenna in this way is called a successive interference cancellation (‘SIC’) scheme. However, when an error occurs in estimating the transmission signal of the (M-1)th transmission antenna, an error also occurs in all following transmission signals estimated by means of the transmission signal of the Mth transmission antenna. Accordingly, it is necessary to propose a scheme for solving the aforementioned problem caused by the characteristics of the SIC reception scheme.